Many of the most useful materials for building and other industrial uses are flammable. This is particularly true of cellulosic materials, such as wood and wood products, paper and cardboard, and textiles from natural plant fibers. Many synthetic materials, such as plastics are also flammable, some highly so. Since these materials have properties that are difficult to duplicate using non-flammable substitutes, much research has focused on how to make these materials less flammable.
Cellulose, such as in wood and paper, is a polysaccharide that burns by a complex oxidative mechanism when subjected to a temperature above about 140.degree. C. The cascading sequence of oxidative reactions includes cleavage of the polysaccharide into its constituent monomers (glucose and glucose derivatives) and oxidative splittings of the glucose rings of the monomers. For example, an intermediate reaction product is levoglucosan which oxidizes further to volatile, flammable compounds and char. The char is believed to be comprised mainly of carbon together with mineral residues. Oxidative cleavages of the chemical bonds comprising the cellulose molecules release large amounts of chemical energy, chiefly in the form of heat and light. The heat produced is also a major factor that perpetuates the cascading progression of oxidative reactions until all the cellulose fuel is ultimately consumed.
Certain phosphorus compounds are very effective as fire retardants for flammable carboniferous substrates apparently because, upon even mild heating, they generate acid residues, such as phosphorous acid, having a high boiling point. These acids react with hot substrate molecules. Their oxidative by-products increase the formation of non-volatile forms of carbon, such as char, from the substrate material, with correspondingly reduced formation of flammable volatiles. As a result, weight reduction of the substrate upon exposure to a burning environment is much reduced.
Although the chemistry of flame retardation and char formation by phosphorus compounds is not entirely clear, a probable mechanism is the increased formation of carbonium ions from the hot substrate due to reaction with phosphorus-containing acids, with attendant aldol-type condensation reactions of the substrate molecules, yielding olefins. The ultimate result is that heat generation is slowed remarkably. Another postulated mechanism is the appreciable reduction or elimination of "afterglow" by blocking the diffusion of oxygen to the hot surface of the substrate.
Certain phosphorus compounds are still recognized as some of the best fire retardants known, especially for cellulosic and many synthetic polymeric substrates. The historically most useful fire-retardant phosphorus compounds include phosphoric acid (H.sub.3 PO.sub.4), monoammonium phosphate (NH.sub.4 H.sub.2 PO.sub.4), diammonium phosphate ((NH.sub.4).sub.2 HPO.sub.4), and ammonium polyphosphate ((NH.sub.4 PO.sub.3).sub.x). Other useful phosphorus compounds include guanidine phosphate, guanylurea phosphate, phosphorylamides, and phosphonitrilic compounds. Generally, a phosphorus content of about 3% or more is effective to impart significant fire-retardancy to cellulosic substrates. Phosphorus seems to be more effective at lower levels than any other fire-retardant element used alone, such as chlorine, bromine, boron, and antimony.
Certain nitrogen compounds, while not generally conferring significant fire retardancy when used alone, seem to participate synergistically with phosphorus in conferring a fire-retardant effect that is greater than the sum of their separate effects. Adding nitrogen often allows the amount of phosphorus in the fire retardant to be decreased. However, more nitrogen is usually needed than the amount of phosphorus omitted to achieve the same result. For example, to impart fire retardancy to wood using monoammonium phosphate, urea is often added to the phosphate solution, where about four to ten parts of urea are added per part of monoammonium phosphate.
Many of the early uses of phosphorus compounds involved the preparation of an aqueous solution of an orthophosphate salt, such as monoammonium or diammonium phosphate, and the application of the solution to, for example, a cellulosic surface such as wood, or the immersion of a substrate material, such as cotton (cellulose fibers) cloth in the solution. The solution is allowed to dry on the material, leaving behind crystals of the phosphate salt on the surface. The main problem with this approach is that the phosphate salt is water soluble. Any subsequent wetting of the treated substrate material causes leaching of the salt, thereby washing away the fire retardancy. Humid environments can also cause leaching.
Another problem with the above approach is that drying of the solution on the substrate sometimes needs to be accelerated by the application of heat. Heat can cause phosphate salts, such as diammonium phosphate, to liberate ammonia, which reduces the fire retardancy of the salt because, as discussed above, ammonia serves as a synergistic nitrogen-containing compound.
Another problem with the above approach is that free phosphate salts are dissociable into ions that can cause structural deterioration of the cellulose. Although such dissociation occurs rapidly in wet conditions, it will also occur on a "dry" surface, which normally has one or more layers of water molecules thereon that originated from the atmosphere. A cellulosic substrate derives a significant portion of its structural integrity by hydrogen bonding between adjacent cellulose molecules. These hydrogen bonds can be disrupted by the incursion of ions (electrostatically charged atomic or molecular species) between the atoms participating in the bonds, which interrupts the bonding interactions between the atoms and ultimately causes the cellulose molecules to separate from one another. Such damage allows penetration of water into and general destruction of the substrate.
Free phosphate salts and low molecular-weight acids, such as phosphoric acid, can also cause delignification of wood by reacting with and cleaving lignins that bind wood fibers together and by cleavage of the cellulose molecules comprising the wood. Such cleavage can ultimately result in a potentially severe loss of structural strength of the wood, especially over a prolonged period of time.
A number of researchers have attempted to solve the problem of phosphate leaching by using fire retardants comprising polymerized forms of phosphate, such as ammonium polyphosphate, which has a variable molecular-weight ranging in the thousands to millions. Such a high molecular-weight is resistant to dissolution and leaching. Unfortunately, ammonium polyphosphate is a solid, crystalline material which is substantially insoluble both in water and in most organic solvents. As a result, to permit application of ammonium polyphosphate to a substrate, the compound must be finely ground into a powder and dispersed in a carrier matrix such as a synthetic polymer, adhesive, or mastic.
One problem with fire-retardant compositions containing ammonium polyphosphate is that, as an insoluble granular compound, ammonium phosphate cannot penetrate into and bond with the substrate. Further, ammonium polyphosphate grains suspended in a carrier can be less effective than a solution of a phosphorus-containing compound in the carrier for ensuring that phosphorus is present throughout the carrier where it is needed to form flame-resistant compounds during combustion. Another problem is the fact that many carriers comprising an organic resin are hydrophobic. As a result, they either covalently bond poorly or not at all to a hydrophilic substrate, such as cellulose, and are consequently vulnerable to peeling and the like, which ultimately results in loss of fire retardancy. Another problem is that the granules of ammonium polyphosphate must usually be individually coated with a layer of synthetic polymer in order to facilitate free-flowing of the granules or to make the material dispersable in a carrier resin. Examples are disclosed in U.S. Pat. Nos. 4,772,642 to Staendeke, 4,701,373 to Fuchs and Staendeke, 4,670,484 to Fuchs and Staendeke, and 4,639,331 to Elsner et al. Nevertheless, ammonium polyphosphate has been particularly useful when dispersed in a resin for making, for example, flame-resistant polyurethane foam, such as disclosed in U.S. Pat. Nos. 4,505,849 to Dany et al. and 4,129,693 to Cenker and Kan. Ammonium polyphosphate is also useful when dispersed in an adhesive used, for example, in the manufacture of flame-resistant chip boards and plywood, such as disclosed in U.S. Pat. No. 4,701,373 to Fuchs and Staendeke.
Other researchers have addressed the phosphate leaching problem by mixing phosphate salts in a polymer resin and applying the mixture to the surface of a substrate. Problems with this approach include the insolubility of the salts in many resins and the persistent tendency of either the non-covalently bound salts or their corresponding ions to leach from the cured resin. Also, free ions of the salts in the polymer are able to penetrate and disrupt the hydrogen bonding between cellulose molecules and cause delignification and cleavage of cellulose molecules, thereby eventually deteriorating the substrate. Further, such resin mixtures are usually poorly soluble in water and other hydrophilic solvents, which can prevent the composition from adhering well to hydrophilic surfaces such as cellulose. Consequently, the applied resin does not withstand the effects of weather and other adverse environmental forces over the useful lifetime of the substrate, resulting in loss of fire-retardancy before expiration of the useful life.
Sometimes, polyols are added to fire-retardant compositions to facilitate a desired polymerization reaction of the composition or to improve flame-retardancy. A polyol is a chemical compound comprising plural hydroxyl moieties. Examples of polyols include sugars, such as glucose, maltose, and arabinose; polyhydric alcohols, such as erythritol, sorbitol, and inositol; polysaccharides, such as starches and dextrins; and synthetic polymers, such as polyvinylalcohol. If the polyol has one or more ring structures, such as trimellitic acid moieties, the polyol may be added to the composition because the rings form flame-resistant crosslinked residues upon combustion. Also, certain polymers, such as polyurethanes, can be formed via reaction of a polyol with, for example, a diisocyanate.
Another reason why polyols have been added to certain flame-retardant formulations is because polyols can facilitate intumescence. Intumescence is the production, upon heating, of a surficial puffy char residue comprised mostly of carbon. The char serves as an insulating layer protecting the underlying substrate from flames. A number of flame-retardant paints, coatings, and mastics are intumescent. An early example is U.S. Pat. No. 2,881,088 to Schulenburg.
A number of current polymeric flame-retardant compositions are thermoplastic upon curing. A "thermoplastic" is comprised of polymer molecules that are either non-crosslinked or are very poorly crosslinked. As a result, even though such a material may feel "solid" at, say, room temperature, heating the material causes softening and eventual melting to a liquid state. Upon cooling, the material returns to a "solid" form. Thermoplastics also tend to be soluble in certain solvents. As a result, thermoplastic compositions can lose their effectiveness in a fire due to their tendency to flow or drip off the substrate upon heating. Also, once the composition melts, or begins to boil in a fire, production of flammable volatiles becomes more likely, which defeats the original purpose of the composition.
The problems discussed above with existing fire-retardant chemicals are particularly acute when such chemicals are applied to cellulosic substrates exposed to weather. For example, shakes and shingles for roofs are often produced from a softwood, such as cedar, which is easily ignited, particularly when dry. Also, building roofs are exposed to one of the most aggressive environments known. This environment includes all the vicissitudes of the weather, including extremes of temperature, sunlight, precipitation, biological growth, and mechanical wear. No fire-retardant compositions are known in the art that will keep substrates, such as wooden shakes and shingles, fire retardant over their useful life. It would be advantageous if such wooden members could be chemically treated to render them fire-resistant, where the fire retardancy would be able to withstand continuous exposure to weather over the useful life of the wooden member.
Hence, there is a need for a fire-retardant formulation particularly suited for and applied as a hydrophilic liquid to cellulosic and other hydrophilic substrates, where the formulation comprises a phosphorus ingredient that becomes non-leachable upon curing the formulation after application to the substrate.
There is also a need for such a fire retardant where the phosphorus-containing ingredient is dissolved in the liquid formulation, rather than suspended in the form of dispersed grains in the liquid formulation.
There is also a need for such a fire-retardant formulation that is substantially water-soluble at the time it is applied as a liquid to the substrate to ensure thorough wetting and penetration of interstices of the substrate and correspondingly superior bonding of the composition to the substrate after curing.
There is also a need for such a composition that chemically reacts with hydrophilic substrate molecules to effect strong covalent bonding of the composition to the substrate molecules and consequent resistance of the composition after curing to effects of weather and other environmental forces.
There is also a need for such a composition that can be applied as a liquid, that cures to a solid, and that has a long shelf life as a liquid before time of use.